linux-kernel - Re: [PATCH 1/2] include: trace: Add SCMI header with trace events

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Date:   Wed, 18 Dec 2019 12:09:00 +0000
From:   Sudeep Holla <sudeep.holla@....com>
To:     Jim Quinlan <james.quinlan@...adcom.com>
Cc:     lukasz.luba@....com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, mingo@...hat.com,
        rostedt@...dmis.org, Sudeep Holla <sudeep.holla@....com>
Subject: Re: [PATCH 1/2] include: trace: Add SCMI header with trace events

On Mon, Dec 16, 2019 at 05:15:54PM -0500, Jim Quinlan wrote:
> From: Lukasz Luba <lukasz.luba@....com>
>
> +
> +TRACE_EVENT(scmi_xfer_begin,
> +	TP_PROTO(u8 id, u8 protocol_id, u16 seq, bool poll),
> +	TP_ARGS(id, protocol_id, seq, poll),
> +
> +	TP_STRUCT__entry(
> +		__field(u8, id)
> +		__field(u8, protocol_id)
> +		__field(u16, seq)
> +		__field(bool, poll)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->id = id;
> +		__entry->protocol_id = protocol_id;
> +		__entry->seq = seq;
> +		__entry->poll = poll;
> +	),
> +
> +	TP_printk("id=%u protocol_id=%u seq=%u poll=%u", __entry->id,
> +		__entry->protocol_id, __entry->seq, __entry->poll)
> +);
> +
> +TRACE_EVENT(scmi_xfer_end,
> +	TP_PROTO(u8 id, u8 protocol_id, u16 seq, u32 status),
> +	TP_ARGS(id, protocol_id, seq, status),
> +
> +	TP_STRUCT__entry(
> +		__field(u8, id)
> +		__field(u8, protocol_id)
> +		__field(u16, seq)
> +		__field(u32, status)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->id = id;
> +		__entry->protocol_id = protocol_id;
> +		__entry->seq = seq;
> +		__entry->status = status;
> +	),
> +
> +	TP_printk("id=%u protocol_id=%u seq=%u status=%u", __entry->id,
> +		__entry->protocol_id, __entry->seq, __entry->status)
> +);
>
> Hello,
>
> When there are multiple messages in the mbox queue, I've found it
> a chore matching up the 'begin' event with the 'end' event for each
> SCMI msg.  The id (command) may not be unique, the proto_id may not be
> unique, and the seq may not be unique.

I agree on id and proto_id part easily and the seq may not be unique
if and only if the previous command has completed.

> The combination of the three may not be unique.

Not 100% sure on that. I remember one of the issue you reported where OS
times out and platform may still be processing it. That's one of the
case where seq id may get re-assigned, but now that's fixed and the
scenario may not happen. I am trying to understand why you think it
is not unique ?

> Would it make sense to assign a monotonically increasing ID to each
> msg so that one can easily match the two events for each msg?

I am not sure if we need to maintain a tracker/counter just for trace
purposes.

> This id could be the result of an atomic increment and
> could be stored in the xfer structure.  Of course, it would be one of
> the values printed out in the events.
>
> Also, would you consider a third event, right after the
> scmi_fetch_response() invocation in scmi_rx_callback()?  I've found
> this to be insightful in situations where we were debugging a timeout.
>
> I'm fine if you elect not to do the above; I just wanted to post
> this for your consideration.
>

I am interested in the scenario we can make use of this and also help
in testing it if we add this. I am not against it but I don't see the
need for it.

--
Regards,
Sudeep